Safety restraints, such as three-point seat belts, five-point harnesses, lap belts, and other restraining devices, are used frequently in motor vehicles to prevent movement of a person's body away from the vehicle. When in motion, a motor vehicle and all articles in and on the motor vehicle have momentum, the product of the article's mass and its velocity. Momentum carries an article in the same direction and with the same speed unless the article is acted upon by an outside force.
During an impact, an external object such as a car, railing, tree, etc. slows or stops the movement of the vehicle, thus reducing or eliminating its momentum. However, the object acts only on the vehicle; the momentum of articles within the vehicle is unaffected, and those articles tend to continue in the same direction and with the same speed just after the impact.
This discrepancy in momentum creates secondary impacts: a coffee cup may strike the window, a book may hit the back seat, or sadly, in some cases, a person may be thrown about within or outside of the vehicle. Safety restraints are used to reduce the effects of this momentum discrepancy. Safety restraints hold a person's body against the car seat and attempt to prevent them from moving out of or sideways from the car seat.
Safety restraints have phenomenally lowered fatality rates associated with motor vehicle accidents. Nevertheless, safety improvements continue to be made. Safety belts were provided with automatic pretensioners and locking retractors. Seat belt warning lights and bells were incorporated into cars. Air bags were developed. In short, a great deal of work has been done to ensure that passengers are protected in the event of an impact.
Unfortunately, while safety restraints prevent injuries, they can cause some as well. To reduce the momentum of a person's body, a seat belt exerts a huge amount of pressure against the body. It is not uncommon for a person to sustain a broken rib in an impact, wherein the rib is broken by the interaction of the body against the belt. In other situations, a person may be badly bruised, or their internal organs may be damaged by the pressure exerted by the seat belt. Some individuals, such as the elderly or those with osteoporosis, are especially at risk of injury from the seat belt.
Of course, it is preferable that all individuals use a safety restraint during an impact rather than go without; injuries from safety restraints are statistically less frequent and less severe than those due to an unprotected crash. However, a way to reduce the damage caused by safety restraints, or to prevent the injuries from them, is needed.